In recent years, CD""s (compact disks), MO (magneto-optical) disks and DVD""s (digital video disks) have been used as optical recording media for recording data such as video images and the like, as well as a recording media for computers. The objective lenses that are used with an optical pick-up device for accomplishing the writing or reading of data signals with these recording media usually have a numerical aperture of about 0.45 for CD""s, 0.5-0.6 for MO disks and 0.6 for DVD""s. In addition, there has been a strong demand for optical pick-up devices to be miniaturized, light weight, and low cost. In order to satisfy these requirements, the objective lens is often made of a synthetic resin material having at least one aspherical surface, or the lens is formed from glass material having at least one aspherical surface.
With the recent rapid growth of data recording capacity, there has been a desire for recording at higher density on recording media, and thus an on-going effort has been made to improve the recording density of optical recording media. This trend in higher density recording makes it necessary for the light collecting spot formed by the objective lens to be smaller. The diameter D of the light collecting spot, where the wavelength of the light source is xcex, and the numerical aperture of the objective lens is NA, is shown by the following Equation (1):
D=kxcex/NAxe2x80x83xe2x80x83Equation (1)
where
k is a constant.
Thus, in order to make the light collecting spot smaller, it is necessary to either shorten the wavelength of the light source, or to use an objective lens having a higher numerical aperture. Shortening the wavelength of the light source requires either a shorter base wavelength semiconductor laser light source or the use of second harmonic generation techniques.
On the other hand, efforts for making an objective lens of higher numerical aperture have continued through the adoption of an objective lens formed of a conventional aspheric single lens element, and this has resulted in a numerical aperture as high as 0.6. However, in further pursuing an objective lens having a higher numerical aperture, there is a problem of the inherent limitations of using a single aspheric lens element. Thus, in Japanese Laid Open Patent Application No. 10-123410, use is made of a two lens element construction in order to achieve an objective lens having numerical aperture of approximately 0.7. However, in the objective lens disclosed in this patent application there is an added complexity both in terms of design and construction in comparison with an objective lens consisting of a single lens element, and this requires an increase in labor and cost. Furthermore, since quality assurance methods for an objective lens having two lens groups (each formed of a single lens element) are generally more complex than for an objective lens using a single lens element, there is generally an increase in the cost of product due solely to the increased cost of inspection of such a two-group objective lens.
The present invention relates to an objective lens used with an optical recording medium that stores information at a high density and, in particular, relates to an objective lens used in a pick up device which accomplishes the writing or reading of data signals on an optical disk, a magneto-optical disk, an optical recording medium of an optical card, and the like at a high density.
A first object of the present invention is to provide an objective lens for high density optical recording wherein aberrations are well-corrected using a two-group, two lens element construction in which the numerical aperture is 0.6 or greater. A second object of the invention is to provide an objective lens having a simple design and construction, which may be easily and inexpensively inspected for quality assurance purposes.